Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
The demand for less expensive and yet more reliable integrated circuit components for use in mobile communication, imaging and high-quality video applications continues to increase rapidly. As a result, integrated circuit manufacturers are requiring greater accuracy in voltage references for such components and devices to meet the design requirements of such myriad emerging applications. Voltage references are generally required to provide a substantially constant output voltage despite gradual or momentary changes in input voltage, output current or temperature. In particular, many designers have utilized band-gap reference circuits due to their ability to provide a stable voltage supply that is insensitive to temperature variations over a wide temperature range.
Band-gap reference circuits are widely used in the analog blocks of an image sensor. An image sensor is configured to work in a power-down mode, a standby mode, and a normal (streaming) mode. In the power-down mode, the image sensor is turned off. After being powered up, the image sensor first enters the standby mode in order to get ready for operations in the normal mode. In the normal mode, the image sensor is configured to receive and output optical images, and a band-gap reference circuit is used for supplying all necessary reference voltages or current. The image sensor may also switch from the normal mode to the standby mode when it had not been used by any program for a predetermined period of time.
FIG. 1 is a timing diagram illustrating the operations of a band-gap reference circuit for use in the analog blocks of an image sensor. Before T1 when the image sensor is powered off (e.g., in power-down mode), the band-gap reference circuit operates in an OFF mode and the band-gap reference voltage VBG is zero. At T1 and after being powered on, the image sensor enters the standby mode and then the normal mode, the prior art band-gap reference circuit operates in an ON mode, and the band-gap reference voltage VBG ramps up in order to reach its nominal level for the normal mode operations of the image sensor. At T5 when the image sensor is powered off (e.g., in power-down mode), the band-gap reference circuit is turned off, and the band-gap reference voltage VBG drops to zero. Between T3 and T4, the image sensor may switch to the standby mode, during which no band-gap reference voltage is required. Since the prior art band-gap reference circuit stills outputs the band-gap reference voltage VBG of the nominal level in the ON mode even if the image sensor temporarily operates in the standby mode, the prior art image sensor may immediately resume normal mode operations after being waked up at T4. However, such prior art band-gap reference circuit consumes significant amount of power.
FIG. 2 is a timing diagram illustrating the operations of another band-gap reference circuit for use in the analog blocks of an image sensor. Before T1 when the image sensor is powered off (e.g., in power-down mode), the band-gap reference circuit operates in an OFF mode, and the band-gap reference voltage VBG is zero. At T1 and after being powered on, the image sensor enters the standby mode and then the normal mode, and the band-gap reference circuit operates in an ON mode. The band-gap reference voltage VBG ramps up in order to reach its nominal level for normal mode operations of the image sensor. At T3 when the image sensor switches to the standby mode, the band-gap reference circuit is turned off, and the band-gap reference voltage VBG drops to zero. At T4 when the image sensor switches back to the normal mode, the band-gap reference circuit operates in the ON mode, and the band-gap reference voltage VBG takes Ts amount of time to ramp up and reach its nominal level for normal mode operations of the image sensor. At T5 when the image sensor is powered off (e.g., in power-down mode), the band-gap reference circuit is turned off, and the band-gap reference voltage VBG drops to zero. Since the prior art band-gap reference circuit is turned off when the image sensor temporally operates in the standby mode, power consumption can be reduced. However, since there are many capacitor loads in the image sensor, it may take a lot of settling time for the image sensor to resume normal mode operations after switching from the standby mode to the normal mode.
It would therefore be desirable to provide a band-gap reference circuit with low power consumption and shortened settling time.